The effect of the addition of three types of sludge (A, V and F) on the slagging behaviour of a brown lignite was investigated. It was evaluated by determining ash fusion temperatures (AFTs) of lignite, sludge samples and coal-sludge blends of 10 and 50 wt% of each sludge, together with their chemical and mineral composition. The addition of sludge to lignite produces a decrease in the AFTs, the extent of which depends on the percentage of sludge added to the lignite, as well as the sludge additives used (organic polyelectrolyte, lime and/or FeCl3 and their phosphorus content As a consequence, the addition of sludge to lignite produces an increase in the slagging tendency. Those lignite-sludge blend ashes whose base to acid ratio (B/A(+P)) ranges from 0.7 to 2.0 have the lowest AFTs. In this range, most of the ash compositions are near to those of SiO2-Al2O3-CaO eutectics. Also, an increase in the phosphorus content of the blends, as well as the formation of amorphous silicates and phosphates, reduce the AFTs. (C) 2014 Elsevier B.V All rights reserved.